Remove BUG_PREFIX from codereview.settings

Since PDFium is no longer using Monorail, it no longer needs to add the
"pdfium:" prefix to Bug: entries in commit message.

Change-Id: I464089bc332def199902aaf4aa9bdeb90587d556
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/119578
Reviewed-by: Thomas Sepez <tsepez@google.com>
Commit-Queue: Lei Zhang <thestig@chromium.org>

codereview.settings

diff --git a/codereview.settings b/codereview.settings
index a23bae5..a673586 100644
--- a/codereview.settings
+++ b/codereview.settings
@@ -1,5 +1,4 @@
 # This file is used by git cl to get repository specific information.
-BUG_PREFIX: pdfium:
 CC_LIST: pdfium-reviews@googlegroups.com
 GERRIT_HOST: True
 PROJECT: pdfium